JP6667535B2 - Imaging device with telecentric optical objective having primary cylindrical lens - Google Patents

Description

The present invention relates to an image pickup apparatus including a telecentric optical objective having a primary cylindrical lens.
In particular, the invention relates to a new type of telecentric optical objective whose main lens consists of single or compound cylindrical lenses, together with a series of technical arrangements, suitable for building imaging systems based on the use of linear cameras. Type of telecentric optical objective.

As is known in the art, a telecentric optical system is an objective system that can form a real image with little perspective effect on an acquisition sensor of a camera, as if the viewpoints were located at infinity. . In this way, the size of the object appears almost constant via the telecentric optical system when the shooting distance changes, at least in the range of the distance given by the depth of field of the telecentric optical system itself.
Accordingly, a major application of the present invention is a vision system based on the use of linear cameras that perform geometric inspection of shape and size by "non-contact" measurements made in image processing technology.

  One of the limitations of the known art is provided by the high cost and difficulty of manufacturing front or primary lenses for current telecentric optics. For such reasons, it is rare to exceed a diameter of 300 mm. The present invention allows such structural limitations to be greatly exceeded. This is because a size larger than 2000 mm can be obtained by processing the cylindrical refraction element. Therefore, the optical combination found can be applied in all cases where the use of telecentric optics is recommended but cannot be realized because the measurement area has a size larger than 300 mm.

  In most cases, telecentric optics are made up of refractive optics, that is, systems consisting only of refractive or spherical lenses. The classical scheme of telecentric optics can be found in FIG. 1 of US Pat. No. 6,324,016, and is shown approximately in FIG. 1 herein with a new reference number. In the diagram of FIG. 1, because of the opaque stop 20 which is inserted at the focal point F of the main lens 12 and has an opening 21, only the ray 18 parallel to the optical axis AO and the ray 19 substantially parallel to the optical axis AO are present. It can be understood how the real image 16 of the observation object 15 is formed through the opening 21. On the other hand, light 17 that is not parallel to the optical axis AO is blocked by the stop 20. The term "real image" means an image formed by the intersection of light rays according to the laws of geometric optics. Telecentric optics are also manufactured industrially by very well known companies such as Carl Zeiss. That is, telecentric optics are known in the art as special objectives and are characterized by forming a real image of a visible object, with near and near parallax substantially canceled. In other words, in simplicity, it can be said that a telecentric optical system provides the appearance of an object as if the viewpoint were located at infinity from the object itself.

U.S. Patent No. 6,324,016

  From a structural point of view, the telecentric optical system is composed of a main converging lens 12 and an optical system assembly 13. The main converging lens 12 is often made up of many simple lenses and forms an achromat or apochromat assembly that is placed in front of the object to be viewed. The optical system assembly 13 is for forming a real image, and is always integrated with the main body of the telecentric optical system 10 at a manufacturing site. This implies that the structural size is much larger while having the same field of view as compared to conventional optical systems. The working distance WD is generally small and in most cases is of the same order of magnitude as the diameter of the front lens.

  It is known to use arrangements to reduce the overall size of telecentric optics, particularly using mirrors to bend the light path, resulting in a smaller overall size. Such an arrangement is part of the invention disclosed in U.S. Patent No. 6,324,016. A catadioptric telecentric solution is also known, in which a parabolic mirror is used instead of the main refractive lens. Such telecentric optics have similar figures to those known for making astronomical telescopes. A better embodiment of a catadioptric telecentric system is obtained by using an off-axis parabolic mirror. Embodiments of catadioptric telecentricity have not been structurally implemented for vision systems, and the telecentric optics available are virtually all refractive.

  Generally, the sensor 14 of the camera 11 is arranged at the image forming point of the telecentric optical system. Then, a digital image of the object 15 placed in the field of view is acquired. The ability to produce images with little or no perspective effects allows telecentric optics to be used in artificial vision systems for geometric and dimensional inspection, resulting in a non-contact metrology management system.

  Obviously, based on the laws of optical physics, in telecentric optics, the size A of the main lens 12 (or mirror in a catadioptric combination) must be greater than or at least equal to the size of the object being imaged. is there. Currently, telecentric optics (Zeiss) are commercially available with a field of view up to 240 × 240 mm, and when combined with a linear camera, a surface with a width of about 300 mm can be obtained. Even if it is possible to obtain a telecentric embodiment with a field of view of the diameter of the main lens larger than 300 mm, its construction cost is very high and is higher than that of other measurement systems, for example mechanical sensors. Get higher. A further limitation on the construction of large telecentric optics is provided by the small size of optical glass for commonly available refractive components. For example, a blank made of optical borosilicate glass Schott BK7 with one side longer than 500 mm is practically impossible to find on the market.

  Embodiments of telecentric optics are known in which the main lens is a Fresnel lens (Light Work LCC) having a field of view of 16 inches, ie 400 mm, and the quality of such optics is, according to the manufacturer, measured and inspected. Is not enough for. Optophysical constraints on the minimum size of the main lens dictate the main configuration limitations of known telecentric optics, and apart from expensive custom embodiments, such objectives for fields of view of 300 mm or more In other words, it is not available.

  Another limitation of the known prior art is caused by the integration of the body of the telecentric objective 10 and the optical assembly to form the real image 16. Such integration implies a fixed magnification, which cannot be corrected and is given by the ratio between the width d of the real image generated by the telecentric optics and the size D of the field of view. For example, if a telecentric objective has a field of view D equal to 120 mm and produces a 12 mm real image on the focal plane, it will have a magnification of 0.1. In order to properly use such a telecentric optical system, it is necessary to connect to a camera with an image sensor having a diagonal line of 12 mm. You cannot find efficient integration due to lack of dimensional correspondence of camera sensors with the right technical features for the application provided, or with the right size sensors, but lack of camera performance there is a possibility. The presence of a market for a large number of cameras with the widest range of sensor types forces manufacturers of telecentric optics to make a very wide catalog available. This is because different apertures (fields of view that can be reached theoretically) are adapted to numerous types of image sensors. For example, to form a complete catalog of telecentric optics with 10 viewing apertures to combine with 20 types of image sensors, the catalog provides 10 x 20 = 200 different product codes Must.

  To solve the disadvantages of the prior art and to obtain these and further objects and advantages, Applicants have studied, experimented with and completed the present invention.

The invention is disclosed and characterized in its independent claims.
The dependent claims disclose other features or variants of the invention.
All claims in the claims are intended to be an integral part of this specification.

  To overcome the inability to use telecentric optics with apertures A greater than 300 mm due to known limitations on the manufacture of large aperture lenses, Applicants have devised a new optical telecentric scheme for linear type image sensors. As shown in FIG. 2, the main lens 23 is of a cylindrical type.

  In such an optical system, the lens 23 must be arranged such that its optical axis is perpendicularly incident on the center point of the image sensor 26. Also, the cylindrical axis AC of the lens must be arranged perpendicular to the linear image sensor 26, and its focal line LF must eventually be adjacent to the opening 21 of the diaphragm 20.

  As is known, image acquisition by a linear camera is performed by making a relative scanning motion between the object (or part) and the camera, oriented along an orthogonal direction to the linear sensor. Thus, the scanning movement occurs along axis Y, parallel to the cylindrical axis AC of the lens.

  The optics scheme with the cylindrical lens of the present invention is strictly telecentric only along the axis provided by the linear sensor, ie the axis parallel to the axis X (see FIGS. 2, 4, 5, 6). From this, the following can be demonstrated. When an image of an object is acquired by a linear camera via the optical system shown in FIG. 2, the image is not affected by the perspective effect due to the scanning of the movement parallel to the axis Y. Such is demonstrated by the following empirical demonstrations.

  As in FIG. 3, a pair of objects or elements 28 and 29 are taken into account. The elements 28, 29 on the plane XY are imaged from a high point on the vertical line with the optical imaging axis parallel to the vertical line via a sensor array device with a traditional optical system that is not a telecentric optical system. The image obtained is the same as that shown in FIG. In such an image, a perspective effect is provided, and the inner wall of the hole 30 and the side wall 31 of the element 29 are partially visible. Furthermore, because of the different thicknesses S1 and S2 of the elements 28 and 29, the visual measurements MVL1 and MVL2 associated with the sizes having the same measurement L in the image shown in FIG. 4 are different, especially with MVL2> MVL1. is there.

  To obtain an image of the elements 28, 29, a camera with a linear sensor and always with common optics is used, and when a scanning movement parallel to the axis Y is performed, an image as shown in FIG. Is obtained. The image is observed to lack the perspective effect along the scanning direction Y, in particular MVL2 = MVL1. This fact is always true for all images obtained by a linear scan, as long as the scan is performed at right angles to the line positioned by the linear sensor and on a plane perpendicular to the optical axis.

  Here, as shown in FIG. 5, in order to cancel the residual perspective effect only along the axis X, it is sufficient to provide the cylindrical lens 23 arranged as shown in FIG. In particular, the lens must be arranged so that its axis AC is orthogonal to the line given by the linear sensor. In this way, the telecentric optics 27 of the present invention produces images lacking perspective effects along X and Y. The main difference to a conventional telecentric objective is that it produces an image instantaneously without the need for a scan that takes a sequence of acquisitions separated in time to produce an image.

  A cylindrical lens such as 23 can be particularly thin in the direction Y. For example, the thickness LL is between 1 and 60 mm. In this way, lens blanks of this type can be easily obtained by water jet cutting from commercially available glass plates of the same thickness with good optical quality. Since a glass plate having a small chromatic dispersion and made of a crown glass having a size of about 6000 × 3000 mm can be found on the market, a cylindrical lens blank having a large aperture can be easily manufactured. Even though commercially available glass plates (as non-limiting examples, Schott BOROFLOAT® 33, Schott SUPREMAX® 33, Corning PYREX®) are not strictly classified as optical glasses, Has optical properties such as transparency and refractive index that are sufficiently excellent for producing the telecentric optical system of the present invention.

  According to one embodiment of the telecentric optical system 27 of the present invention, to solve the problem that a cylindrical lens is affected by a cylindrical aberration in the same way that a spherical lens is affected by a spherical aberration, the lens 23 has one side. The surface is formed with a suitable “non-cylindrical (non-cylindrical)” profile (FIG. 9), which advantageously gives the deflection angle of the lens 23. The non-cylindrical contours calculated by the Applicant have proven to be excellent in both "ray tracing" simulations and the two generated prototypes. That is, they were two cylindrical lenses with an opening of 500 mm and a focal point of 1000 mm, and in the inspection performed, there was substantially no cylindrical aberration.

  As is known in the art, cylindrical lenses, ie, lenses defined by a cylindrical surface, have cylindrical aberration. The cylindrical aberration exists in the dispersion around the focal point of the image point of the light beam incident on the lens parallel to the optical axis of the lens. Such an aberration is equivalent to the spherical aberration of a spherical lens, ie, a lens separated by a spherical surface. Geometries that allow removal of cylindrical aberration can be calculated using known numerical calculation programs.

  An advantageous solution is to employ a biconvex contour (FIG. 9) for the main lens 23, in which the inner contour 40 is non-cylindrical, while the outer contour 41 is cylindrical. And has a radius that maximizes the distortion of the telecentric objective 27. A further improvement is provided by employing a non-cylindrical profile for the outer profile 41 as well. The non-cylindrical contour is calculated such that the main lens 23 has a geometric distortion equal to and opposite to that of the optical assembly for forming the real image 16. In this way, it is possible to obtain a telecentric optical system that has great advantages for non-contact optical checking applications without geometric distortion in addition to eliminating perspective parallax.

  Further embodiments of the present invention may be obtained by forming the cylindrical lens 23 as a composition of two or more elements to form an achromat or apochromat type optical assembly.

  A very advantageous embodiment of the telecentric objective 27 of the present invention replaces the optical assembly 13 for forming the real image 16 with a well-known photographic objective 33 of a known type performing the same function. is there. In order to introduce such a deformation, in the optical arrangement of the present invention, a mechanical mounting element 32 must be added between the photographic objective and the camera body 25.

  In such a variant, the adjustable stop of the photographic optical system 33 is replaced by the stop 20 with the opening 21 of the first above-described embodiment, thereby obtaining a simplification of the structure.

  By properly selecting the focus of the objective 33, the aperture A of the main lens can be adapted to different types of camera image acquisition sensors. By introducing the interchangeability of the optical assembly 13 for imaging through the insertion of a photographic objective, the number of telecentric product codes for a given size of field of view is greatly reduced, thereby reducing the number of prior art product codes. Overcome the limitations of growth.

  Further improvements can be obtained by inserting a zoom objective as the photographic optical system 33 for forming the real image 16. This makes it possible to optimally adapt the plurality of types of the image sensor 26 in either the maximum aperture A of the front lens 23 or a desired field of view (smaller than the aperture A).

  The advantages of using commercially available photographic optics 33 as an assembly for forming a real image over the prior art are several and are summarized below.

The possibility of adapting a plurality of cameras with different types of image sensors to the opening A of the main lens 23 or obtaining a desired field of view smaller than A to maximize the resolution of the acquired image.
-Possibility to facilitate focus adjustment.
The possibility of adjusting the depth of field by acting on the aperture of the optical system 33;
The possibility of having a single product code for a given aperture of the telecentric optics of the invention.

  These and other features of the present invention will become apparent from the following description of embodiments, provided by way of non-limiting example, with reference to the accompanying drawings. The accompanying drawings include:

FIG. 1 is a schematic diagram of a conventional telecentric objective. FIG. 2 shows the cylindrical main lens 23 of the present invention according to the first embodiment. According to the present embodiment, the cylindrical lens has a non-cylindrical contour for removing cylindrical aberration. Further, the lens 23 is composed of many elements for reducing other forms of aberration such as chromatic aberration. FIG. 3 illustrates the operation of the invention described above. FIG. 4 illustrates the operation of the invention described above. FIG. 5 illustrates the operation of the invention described above. FIG. 6 shows a further embodiment of the present invention that includes a photographic optical system 33 of a known type as an assembly for forming the real image 16 on the sensor 26 of the camera 24. This figure includes an adapter fitting at the connection between the optical system 33 and the camera body 25. FIG. 7 shows, by way of non-limiting example, a general apparatus for acquiring and processing images, including the present invention, known in the art as a "viewing system", where the apparatus has near-far parallax. Geometric, dimensional and metrical checks are performed using the characteristics of producing missing images. FIG. 8 shows, by way of non-limiting example, a general image acquisition and processing device having a wide field of view. In this figure, two mirrors having a first surface are inserted and the optical path is folded into three parts, which can encompass a wide range of devices including the present invention. FIG. 9 shows, by way of non-limiting example, the application of the optics of the invention as a system for producing a one-dimensional parallel beam, which is suitable for use in a 3D image acquisition solution, Has better performance compared to the known system.

  The term "mirror having a first surface" means a mirror with a reflective surface arranged in front. As a result, the incident light and the reflected light do not cross the glass constituting the mirror body.

  It should be noted that in all figures, the elements supporting the depicted elements have been omitted in order to make the figures clearer and because the enclosing and supporting elements are not innovative with respect to the invention. Must. In any case, support elements and inclusion elements are necessary to actually practice the invention, and those skilled in the art will be able to practice the invention based on all information contained herein.

  Obviously, the telecentric optics 27 with the cylindrical lens 23 described herein can be modified and / or added without departing from the scope of the present invention.

  Although the invention has been described with reference to specific examples, those skilled in the art will recognize many other aspects of the telecentric optics having features as described in the claims and within the scope defined thereby. Could certainly be made.

FIG. 7 schematically illustrates a portion of an apparatus 35 for acquiring images of various types of elements placed in an industrial mass production environment. This device comprises the telecentric optics 27 of the present invention, an illumination system 34, a belt-type transport system 38, and a camera 24. If the cylindrical lens 23 is of a simple type, the illumination 34 lacks chromaticity in a single color of a narrow spectral band, for example, in a wavelength band of 40 nm, and therefore a clear image can be obtained. Therefore, it is advantageous to use a light emitter that has an LED light source and emits light of a single color, or a light emitter based on a fluorescent tube (sodium, argon, neon, etc.) that emits substantially monochromatic light. The belt moves the part 37. Following a predetermined advance of belt 38, an encoder coupled to belt 38 via pulleys (not shown) generates a series of pulses to cause camera 24 to acquire a number of image lines. Camera signals, including image lines formed as a series of binary words (bytes), are captured into the processor's RAM storage via a communication port. The set of lines forms a digital image of the objective system 27.
In such an image, the object appears to lack parallax. Thus, by processing the image itself, it is possible to extract a series of measurements of the component 37. For example, the number and position of holes, the diameter of each hole, and the component size.

  The use of the cylindrical lens 23 to form the present invention is advantageous over the prior art in that the volume of the resulting telecentric optical system is reduced, especially along the Y direction. The relatively easy creation of large cylindrical lenses (e.g., with an aperture A equal to one meter) makes it possible in any case to make the invention very large along the axis Z and parallel to the optical axis in FIG. .

  To reduce the size along the axis Z, the optical path can be folded a certain number of times using a plane mirror, as shown in FIG. For example, using two mirrors allows the optical path to be split into three parts, greatly reducing the overall spatial size of the present invention. According to applicants' experience, it is desirable to use a first surface mirror to avoid halos and the formation of double images. Further, it is desirable not to introduce three or more mirrors in order to prevent deterioration / attenuation of the image quality of the object to be acquired.

  Referring to FIG. 9, the new optics shown therein can be used to generate perfectly parallel emission lines, ie, lines composed of parallel light propagation.

  Known techniques for 3D mapping images include the use of a line emitter (typically a laser) and the use of a camera that shoots the line projected onto the object to be 3D mapped from a predetermined angle. Based. Typically, the optical axis of the emission line is perpendicular to the plane of interest, while the optical axis of the 3D profile acquisition camera is tilted 20-40 degrees with respect to the normal to that plane. In this case, an important prior art limitation is the divergence of the light rays forming the emission line. Due to such divergence, light cannot reach the bottom of the closed recess 46 located outside the optical axis. The further away from the optical axis of the light-emitting line, the less the light-emitting line can reach the bottom of a depression (eg, a hole). FIG. 9 shows a mechanism by which the collimation of the emission line in the light ray 45 parallel to the optical axis can be obtained by using the optical system of the present invention. Such light rays can reach the bottom of the blind hole 46 and form the emission line 47 regardless of the depth of the hole and independent of the distance from the optical axis. In contrast, without the lens 23 of the present invention, it can be seen why the divergent ray 44 generated by the emission line generator 43 (comprising the light source 42 and the optical assembly 33) cannot reach the bottom of a similar blind hole.

If an array camera combined with yet another telecentric optics of the present invention is used to capture the emission lines projected by the 3D mapped object, conventional 3D scanners typically map. Closed depressions that are quite deep for diameters that cannot be made (eg, holes 3 mm in diameter and 15 mm deep) can also be mapped. Thus, the present invention can significantly improve the known 3D scanning system.

Claims (11)

An imaging device for acquiring an image of a target object (15, 37) without perspective parallax,
A lighting device (34);
An operation system (38) for the object (15, 37);
A linear camera (24) having a linear image sensor (26);
A telecentric optical objective (27);
The telecentric optical objective (27) for forming a real image (16) of the object (15, 37) arranged between the linear camera (24) and the object (15, 37). An optical assembly (13), a stop (20) disposed between the optical assembly (13) and the object (15, 37) and having an opening (21), and the stop 20 and the object ( 15, 37), and a main lens comprising a cylindrical lens (23).
The optical axis (AO) of the cylindrical lens (23) passing through the aperture (21) of the stop (20) and the optical assembly (13) is at the center point of the image sensor (26), and in the plane thereof. Cross vertically,
A cylindrical axis (AC) of the cylindrical lens (23) is orthogonal to the optical axis (AO), which is a main axis of the image sensor (26) ;
The cylindrical lens (23) has a pair of planes parallel to the z-direction extending in the optical axis (AO) direction due to the edge of the glass plate, and has a non-cylindrical shape, ie, a cylindrical surface having a constant radius. Having at least one optical separation surface having a shape different from the shape,
The imaging device, wherein the non-cylindrical shape is calculated to eliminate cylindrical aberration of the lens (23) . An apparatus capable of producing a one-dimensional parallel beam and having said telecentric optical objective (27),
A photographic objective system (33) having an aperture (20) with an opening (21) for forming a real image of the object arranged between a light source (42) and the object. A system (33), and a main lens including a cylindrical lens (23) disposed between the photographic objective system (33) and the object;
The cylindrical lens (23) has a biconvex shape,
The inner contour (40) of the cylindrical lens (23) has a non-cylindrical shape, that is, a shape different from the shape of a cylindrical surface having a constant radius, while the outer contour (41) of the cylindrical lens (23). Has a cylindrical shape with a radius that maximizes the distortion of the telecentric objective (27),
The optical axis of the cylindrical lens (23) passing through the photographic objective (33) coincides with the axis of the light source (42);
The device according to claim 1, wherein a cylindrical axis of the cylindrical lens (23) is orthogonal to the optical axis. The outer contour (41) also has a non-cylindrical shape, the non-cylindrical shape being such that the cylindrical lens (23) is equal and opposite to the geometric distortion of the photographic objective (33) for forming a real image. The apparatus of claim 2 , wherein the apparatus is calculated to have a geometric distortion of The device according to any one of claims 1 to 3 , wherein the cylindrical lens (23) is made up of two or more cylindrical lenses, forming an achromat or an apochromat assembly. The optics assembly (13) for forming the real image (16) has a fixed focus (33) and is fitted to the body (25) of the camera (24) via a fixture (32). Composed of a system,
The fixed focus is selected such that the resulting viewing angle (Phi) for the type of sensor (26) covers at least 80% of the aperture (A) of the main lens (23). 1, apparatus according to any one of claims 4. The optical assembly (13) for forming the real image (16) comprises a photographic objective having a variable focus and adapted via a fixture (32) to the body (25) of a camera (24). And
Range of the focal length of said photographic objective system, the main the opening of the lens viewing angle covering 100% of (A) (Phi) is selected so as to obtain, according to claim 1, one of the claims 4 An apparatus according to claim 1. The lighting device (34) includes a light emitting source having a narrow spectral band, according to any one of claims 1 and claims 4-6. The apparatus of claim 7 , wherein the lighting device (34) comprises a single color LED light source. Having a 300mm larger field of view, according to any one of claims 1-8. Only approximately equal coefficient number obtained by adding 1 to the number of mirrors used, comprising at least one mirror adapted to reduce the overall size of the device (39), one of the claims 1-9 An apparatus according to claim 1. The device according to claim 10 , wherein the mirror (39) is of the type having a first surface.

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